For many years, the natives of South and Central America found use for a latex obtained from the barks of certain trees. Said latex was often a cis- or trans-1,4-polyisoprene and it was recovered from rubber trees via a tapping process similar to the one used to recover maple sugar.
In the early nineteenth century, individuals such as Hancock in England and Goodyear in the United States discovered that mixing natural rubber with sulfur yielded a moldable composition which could be vulcanized and converted to a variety of commercial products. Such commercial products included waterproof coats and boots as well as solid tires for transportation vehicles.
In the period between World Wars I and II, the development of synthetic rubbers was pursued; especially in the United States. During this period, a program was instituted to produce synthetic rubbers such as styrene-butadiene (SBR) which resulted in a decrease in America's dependence on rubber imports as well as an increase in synthetic polymer production.
Presently, synthetic polymers such as acrylonitrile-butadiene-styrene copolymers (ABS) are produced by grafting monomers onto a rubbery polymer backbone in a latex. In the case of ABS, acrylonitrile and styrene are frequently grafted onto rubbery polybutadiene and/or a rubbery styrene-butadiene copolymer in a latex. Prior to grafting, said latex of rubbery polybutadiene is generally admixed with the monomers to be grafted thereon and the mixture is usually agitated for a period of up to several hours to emulsify the monomers and to enable the rubbery polymer to absorb them before initiating the grafting reaction.
Typically, the above-mentioned rubbery backbone polymer is prepared via processes that subject, for instance, butadiene to catalysts of the alfin or amylsodium type. More typically, polybutadiene latices are produced by emulsion polymerization techniques that employ an aqueous emulsion of butadiene, an anionic emulsifier and a free radical initiator. The polymer backbones produced by the processes described above consist of usually about 80% by weight of 1,4-polybutadiene units (1,4-enrichment) and about 20% by weight of 1,2-polybutadiene units based on total weight of the polymer in the latex. Moreover, said 1,2-polybutadiene units are present as isolated units or as blocks in the polymer backbone, and the blocks are often less than four butadiene units in length.
It has been of increasing interest to prepare polybutadiene latices, as well as homologs derived therefrom, that comprise high 1,2-enrichment. This is true since it is expected that high 1,2-enrichment will enhance the activity of the polymer latices during conventional grafting reactions.
Accordingly, the instant invention is based on the discovery of novel polybutadiene latices, as well as homologs derived therefrom. Said novel latices comprise greater than about 20% by weight and up to about 50% by weight of 1,2 units (1,2-enrichment) based on total weight of the polymer in the latex, wherein about 25% to about 75% by weight of all 1,2 units are present in the polymer backbone and about 75% to about 25% by weight of all 1,2 units are blocks grafted onto the polymer backbone. Furthermore, the 1,2 blocks grafted onto the polymer backbone are greater than about 5 diene units in length and preferably about 20 to about 100 diene units in length.
A second aspect disclosed herein is a novel process for producing the above-mentioned novel latices.